1. Field of the Invention
The present invention generally relates to electromagnetic lenses and yokes and, more particularly, to a simplified method for winding the coils of electromagnetic lenses and yokes.
2. Description of the Prior Art
Yokes and electromagnetic lenses are widely used in electron beam tools, electron microscopes, and cathode ray tubes (CRTs). Yokes and lenses employing toroidal magnetic deflection coils are commonly used electron beam lithography systems for focusing an electron beam on to a substrate for submicron patterning of semiconductor devices.
U.S. Pat. No. 4,251,728 to Pfeiffer shows an example of a toroidal magnetic deflection yoke. The operation of such a yoke is fairly straightforward. Opposing rectangular windings are arranged in a circle to form a toroid. For example, FIG. 1A shows one pair of opposing coils, 10 and 10', arranged at opposite sides of a circle 12 forming a toroidal yoke. Typically, one of these coils controls the electron beam deflection in the Y direction and the other coil in the X direction. In operation, a deflection current is passed through the coils from point A to point B through interconnection 14. The deflection current travels in a direction specified by the arrows I.sub.o and I.sub.i. Magnetic fields 16 are created in each leg of the coils in a direction prescribed by the well-known right-hand rule. An electron beam 18 traveling through the center of the toroid, between opposing legs of the coils, is influenced by the aggregate of the magnetic fields 16 and deflected to a new path 18'.
FIG. 1B a shows a sectional view of the windings shown in FIG. 1A taken along line 2-2'. It can be seen that the electron beam 18 can be deflected in either direction by controlling the deflection currents in the coils 10 and 10'.
Presently, electromagnetic coils are formed by winding wire into multiple radial cut grooves cut on a plastic form. The method used for winding the form to make the coils is characterized as being difficult and time consuming. Proper winding requires alternating between the X and the Y windings. As the number of radial cut grooves and the number of turns increases, so does the degree of difficulty and the time involved.
FIG. 2 shows a top view of a traditional toroidal yoke. A plastic form having slots numbered from 1 to 19, starting at the twelve o'clock position and counting counter clockwise, form both the X and Y coil axes which make up the toroidal yoke. FIG. 3 shows the toroidal yoke of FIG. 2, opened up along line 3-3'. As is readily apparent, the winding process is quite involved. Referring now collectively to FIGS. 2, 3, and 4A-D, the recommended winding procedure is as follows:
Step 1. Take three spools no larger than 1.5 inches in diameter and wrap 130 inches of 20 AWG wire onto each of two spools. Label the first spool "X1" and the second spool "X2". Wrap 240 inches of 20 AWG wire onto the third spool and label it spool "Y".
Step 2. Take the spool labeled "X2" and unwrap three inches of wire and label the end "X2 Start". Beginning at slot No. 19 wrap according to the view shown in FIG. 4D. Tape the wire to the form.
Step 3. Take the spool labeled "Y" and unwrap six inches of the wire and label the end "Y Start". Beginning at slot No. 18, wrap according to the winding style shown in FIG. 4A. Continue to wrap in a counter clockwise direction to slot No. 20 according to the winding style shown in FIG. 1A. Repeat this for slots Nos. 1 and 2. Tape the wire to the form.
Step 4. Take the "X2" spool and go counter clockwise to slot No. 17 and wrap according to the winding style shown in FIG. 4C. Repeat this for slots Nos. 16 and 15. Tape the wire to the form.
Step 5. Take the "X1" spool and unwrap six inches of wire and label the end "X Start". Beginning at slot No. 3, wrap according to the winding style shown in Figure shown in FIG. 4D. Tape the wire to the form.
Step 6. Take the "Y" spool and go counter clockwise to slot No. 4 wrapping according to the style shown in FIG. 4A. Now bring the spool clockwise to slot No. 14 and wrap according to the winding style shown in FIG. 4B. Tape the wire to the form.
Step 7. Take the "X2" spool and go clockwise to slot No. 13 wrapping according to the winding style shown in FIG. 4C. Bring the spool counter clockwise to slot No. 1. Remove the wire from the spool and label it "X Return".
Step 8. Take the "Y" spool and go counter clockwise to slot No. 12 wrapping according to the winding style shown in FIG. 4B. Repeat this for slot Nos. 11 and 10. Put a spacer into slot No. 9 at this point. The spacer should be two layers of wire, three rows wide. Take the "Y" spool clockwise to slot No. 8 and wrap according to the winding style shown in FIG. 4B. Take the spool counter clockwise to slot No. 16. Remove the wire from the spool an label it "Y return".
Step 9. Take the "X1" spool and go counter clockwise to slot No. 5 wrapping according to the winding style shown in FIG. 4D. Repeat this for slots Nos. 6 and 7. Remove the spacers from slot No. 9. Remove the wire from the spool. Take the wire and thread it into slot No. 9 according to the winding style shown in FIG. 4D. Take the end of the wire clockwise to the space between slot No. 1 and slot No. 20. Strip both this and the end of the wire labeled "X2 Start" about 0.38 inches apart. Put a one inch length of heat shrink tubing onto the wire labeled "X1" and overlap the wire ends 0.38 inches. Solder these wires together and cover joint with shrink tubing.
Step 10. Take "X Start" to slot No. 16 and label it "X Drive". Twist it together with "X Return".
Step 11. Take "Y" to slot No. 1 and label it "Y Drive". Twist it together lightly with "Y return".
This process is time consuming and prone to mistakes. For instance, as the number of turns goes up, so does the degree of difficulty and time involved. Keeping uniform tension on the wires and keeping the wire straight is also a difficult task. Any slack or tangling results in an unbalanced, non-functioning yoke. Cooling is also a problem with this type of wound yoke since, at best, only about 30% of the coil area facing away from the plastic form is accessible to a cooling medium. This results in poor, unbalanced cooling. For high power applications this situation is unacceptable since the increased amounts of heat that is generated will adversely effect yoke performance.